MIRACLES OF SCIENCE 



from the focus of gravitation at the earth's center. 

 Gravitation, it will be recalled, decreases with the 

 square of the distance, and although the altitudes at 

 which experiments may be conducted are at most 

 only a few miles, as contrasted with the four thou- 

 sand mile radius of the earth, yet pendulums may 

 be constructed of sufficient delicacy to mark the dif- 

 ference; and of course the theoretical difference be- 

 tween the earth's gravitation pull at sea level and at 

 an altitude of let us say five miles is a matter of 

 simple computation. Where, however, a pendulum 

 test is made on a high plateau, the disturbing influ- 

 ence of the mass of rock making up the plateau is 

 measureable, because this mass tugs at the pendu- 

 lum and makes it swing a little faster than it theoret- 

 ically should do. The amount of variation deter- 

 mines the relation between the mass of the plateau 

 and the total mass of the earth itself; and from this 

 proportion the total mass 'of the earth may be calcu- 

 lated, if the mass of the plateau is known. 



Another method of utilizing a localized mass, 

 different only in detail from that just outlined, is 

 to operate with a plumb line on either side of a large 

 hill or mountain of the "hogback" type, noting the 

 extent to which the plumb line is pulled away from 

 the vertical by the attraction of the mountain. As- 

 tronomical observations determining the true ver- 

 tical, and the actual deflection is matter of observa- 

 tion, but it is necessary to survey the mountain 

 accurately and determine as closely as possible the 

 precise character of the rocks of which it is com- 

 posed. Classical experiments of this type were made 



62 



